In advanced integrated circuit technology, titanium-tungsten and tungsten films are often used to create transistor interconnects. To form the interconnects, a layer of titanium-tungsten or tungsten may be deposited by chemical vapor deposition techniques (CVD) or sputtered over an underlayer which is generally made of either titanium disilicide, titanium nitride or silicon dioxide. A titanium nitride underlayer, in particular, provides a good base to which tungsten adheres. To create the interconnects it is necessary to remove part of the titanium-tungsten or tungsten layer without significantly affecting the underlayer. Several methods for doing this have been proposed.
One method is to use plasma etching in which a glow discharge is utilized to produce chemically reactive species from a relatively inert molecular gas. The etching gas is selected so as to generate species which react chemically with the material to be etched, and whose reaction product with the etched material is volatile. Examples of known plasma etching processes are given below.
In U.S. Pat. Nos. 4,842,687 and 4,842,676, methods of etching tungsten films are disclosed using a gas mixture which includes a fluorine source (SF.sub.6), a bromine source (HBr) and a hydrocarbon source such as methane. The etching methods described in these patents are selective of silicon dioxide and photoresist layers. Etch rate ratios of 2-3 to 1 are obtained using these methods.
Daubenspeck et al. U.S. Pat. No. 4,836,887 describes a selective etching method for tungsten using a fluorinated gas, an oxidant and 15-20 percent of a chlorofluorocarbon gas. This method provides a maximum etching rate ratio between tungsten and photoresist of less than 2.5. It is also suggested that high etch rates for silicon containing films can be obtained using this etching method.
In U.S. Pat. No. 4,836,886, Daubenspeck describes a method for etching tungsten and silicon with trifluorochloromethane and at least 50 percent oxygen. High etch rates are achieved for both tungsten and silicon with this method. Again, a maximum etch rate ratio of less than 2.5 was achieved with this method with respect to tungsten and photoresist.
U.S. Pat. No. 4,797,178 describes plasma cleaning methods in which a metallic layer, e.g. tungsten, is etched using a large mass inert gas (such as argon) and CF.sub.4 and O.sub.2. The large mass gas mechanically removes any polymerized fluorocarbons that form on the surface and which tend to block etching.
U.S. Pat. No. 4,786,360 describes a selective etching method for tungsten using a binary mixture of chlorine gas and 25-45 percent oxygen. This etching method provides high etch rates for tungsten and titanium nitride and is highly selective with respect to dielectrics such as boro-phososilicate glass (BPSG), phososilicate glass (PSG) and polymers such as polyimide.
Tsang in U.S. Pat. No. 4,713,141 discloses an anisotropic plasma etching method for tungsten. With this method, a gas mixture of SF.sub.6 and Cl.sub.2 is used wherein the concentration of chlorine is between 20-50%.
Sachdev et al., in U.S. Pat. No. 4,692,205 describe an ion etch method using a silicon polyimide etch barrier and CF.sub.4 and O.sub.2 reactive ion etchants. The CF.sub.4 /O.sub.2 gas mixture effectively etches the silicon polyimide barrier layer under reactive ion etching conditions.
Douglas in U.S. Pat. No. 4,675,073 teaches an etching method for TiN which has improved selectivity with respect to TiSi.sub.2. The method uses a mild fluorine source, such as CF.sub.4 ; a reducing electrode, such as silicon or graphite to scavenge fluorine; a relatively low flow rate; and a substrate temperature between 60.degree. C. and 100.degree. C.
While the above etching methods offer good etch rates for tungsten and tungsten containing films, these methods tend to etch silicon containing films as well. Indeed, with some methods the silicon films are etched faster than the metal films. Not only do these prior art methods etch silicon containing films, these methods also tend to significantly etch the underlayers made of titanium disilicide, titanium nitride and silicon dioxide. Thus, often etch barriers are used, or etch times are carefully controlled when etching tungsten containing films with the prior art methods so as to not over-etch these underlayers.
In view of the above, it is an object of the present invention to provide a method for etching tungsten and titanium-tungsten films which is highly selective of the underlayers made of titanium disilicide, titanium nitride and silicon dioxide.